Treatment of sleep disordered breathing (SDB), such as obstructive sleep apnea (OSA), by continuous positive airway pressure (CPAP) flow generator systems involves the continuous delivery of air (or other breathable gas) pressurized above atmospheric pressure to the airways of a human or other mammalian patient via a conduit and a mask. Typically, the mask fits over the mouth and/or nose of the patient. Pressurized air flows to the mask and to the airways of the patient via the nose and/or mouth. As the patient exhales, carbon dioxide gas may collect in the mask. A gas washout vent on the mask or conduit discharges the exhaled gas from the mask to atmosphere.
The washout vent is normally located on the mask, such as on the frame, on an elbow connected to the mask, or near the mask in the gas delivery conduit coupled to the mask. The washout of gas through the vent to the atmosphere removes exhaled gases to prevent carbon dioxide build-up, and hence “rebreathing”, which represent a health risk to the mask wearer. Adequate gas washout is achieved by selecting a vent size and configuration that allows a minimum safe washout flow at a low operating CPAP pressure, which typically can be as low as 4 cm H2O for adults and 2 cm H2O for children. A typical range is between 2-30 cm H2O, while a typical treatment pressure is about 8-12 cm H2O for adults.
Noise is a significant issue in CPAP treatment for the patient and/or the patient's bed partner. Excessive noise can lead to patients being non-compliant with the CPAP therapy. One source of noise is the exhaust through the vent in the mask or conduit. The flow of gas through the vent creates noise as it exits to and interacts with the atmosphere. Noise can adversely affect patient and bed-partner comfort, depending on both the magnitude and character of the noise. Further, bi-level gas delivery therapies tend to generate more noise than do constant level gas delivery regimes. This is thought to be due to the extra turbulence created by the gas accelerating and decelerating as it cycles between relatively low and relatively high pressures in the bi-level gas delivery systems.
“Air Jetting” out of the vents is also a significant issue. Air jetting, or lack of diffusion in the vent, involves a high-velocity jet stream of exhaust gases blowing onto obstacles (such as bedding, bed partner, or even onto the mask wearer themselves). This not only causes a significant increase in noise due to a sudden change in velocity of the exhausted air, but the high-velocity jet stream also creates great discomfort for the bed partner or mask wearer as a result of “wind chill”. Accordingly, a plurality of diffuse vent holes may be used.
For example, FIG. 19 illustrates a prior art elbow 260 having a gas washout vent 262, which includes a plurality of diffuse vent holes 266. This gas washout vent 262 is further described in U.S. patent publication 2009/0050156, published Feb. 26, 2009, which is incorporated herein by reference in its entirety. FIGS. 20 and 21 illustrate a perspective view and a cross-sectional view, respectively, of the gas washout vent 262.
The gas washout vent 262 includes a first or bottom wall 264, a second or top wall 268, a plurality of supports 270 extending between the first wall 264 and the second wall 268, and a plurality of vent holes 266 formed between the supports 270. The supports 270 are disposed on an outer edge of the second wall 268 extending outward from the second wall 268, and are disposed on an upper surface of the first wall 264. The vent holes 266 form passageways allowing escape of the washout gas.
While the vent arrangement of FIGS. 19-21 works in a satisfactory manner for its intended purpose, a need has developed to even better vent to deal with the possibility of moisture in the exhausted air. The moisture may tend to bead around the vent holes, in particular on the surfaces immediately outside of the vent holes. The beads of water may vibrate as the exhaust gases rush through the vent holes and past the beads of water, causing a whistling noise. Additionally, the water beads may cause a partial obstruction of the vent holes further contributing to vent noise.